Photonic devices, such as semiconductor lasers, optical modulators and optical amplifiers are widely used in modem telecommunication systems. It is desirable to monitor the optical output of such photonic devices on chip. This is especially desirable when multiple devices are integrated onto one chip, and more than one optical device has to be monitored.
However, it is problematic to control or monitor the output of an optical device output because the gain of a laser or amplifier can be affected by a number of factors, including:    i) environmental effects, such as temperature, humidity, changes in wavelength and polarisation etc;    ii) device degradation, due to crystalline defects, deterioration of contacts, etc; and    iii) misalignment of optical coupling elements due to shock, strain, etc.
At present, bulk detectors and couplers are used to monitor and control a semiconductor laser or amplifier, but these prove to be expensive, lossy and impractical for large scale monolithic integration.
For a semiconductor laser, a photodetector can be positioned at the back facet of the laser. The facets of a semiconductor laser are typically coated with a highly reflective (HR) coating, having a reflection coefficient, R, of up to ˜95% on the back facet and an anti-reflection (AR) coating with R˜5% on the front facet. The photodetector can measure the light escaping from the back facet (R˜95%) and hence monitor the device.
For a semiconductor optical amplifier, no facet is available for monitoring by a photodetector, since both the front and back facets are employed for ingress and egress of optical radiation. Therefore, one solution, as taught in U.S. Pat. No. 5,134,671, is to employ an integrated branching waveguide, such as a Y-junction waveguide, to tap off a fraction of the output power to feed to a photodetector, to monitor the amplifier.
U.S. Pat. No. 5,134,671 describes a monolithic integrated optical amplifier and photodetector. The optical amplifier and photodetector are integrated on the same substrate, the photodetector being optically coupled to the optical amplifier via a branching waveguide having low radiative loss and low back reflectivity. This is achieved with a difficult manufacturing process to form the Y-shaped waveguide with a branch of the waveguide having a gradual decrease in the effective refractive index such as to decrease the difference between the refractive indices at the optical interface of the truncated wedge tip to avoid optical coupling of the amplifier.
Due to fabrication/device limitations, practical Y-junction waveguides have truncated wedge tips. See, for example, Sasaki et. al. Electronic Letters, Vol. 17, No. 3, pp 136-8 (1989). However, a blunted Y-junction tip, which inhibits a substantial amount of optical back-reflection to the optical amplifier, restricts the monolithic integration of a coupled optical amplifier and monitoring photodetector.
A 1.3 μm laser with an integrated power monitor using a directional coupling optical power tap is described in U. Koren et al, IEEE Photonics Tech. Letters, Vol. 8, No. 3, p364 (1996). This work describes a Y-junction optical tap using a passive dual waveguide directional coupler next to the back HR facet of the cavity.
The disadvantage with the process described is that four growth steps are required to construct the device, including an overgrowth to deposit the passive waveguide region. The different growth steps considerably increase the device fabrication difficulty, hence reduce yields and increase costs.